1. Technical Field
The present invention relates to a method of manufacturing porous sintered reaction-bonded silicon nitride ceramics and porous sintered reaction-bonded silicon nitride ceramics manufactured thereby. More particularly, the present invention relates to a method of manufacturing a porous sintered reaction-bonded silicon nitride ceramics, which controls a pore structure so that the specific surface area of pores is increased to improve capturing performance and coarse pores are formed to enhance air permeability, and to a porous sintered reaction-bonded silicon nitride ceramics manufactured using the same.
2. Description of the Related Art
Silicon nitride based materials are superior in terms of strength, toughness, impact resistance, heat resistance and corrosion resistance despite being lightweight, and thus have been widely used in fields requiring good thermo-mechanical properties and chemical resistance.
Conventionally, silicon carbide based porous materials have been mainly utilized in fields requiring thermo-mechanical properties and chemical resistance, but are problematic because silicon carbide has low thermal shock resistance and high hardness, and thus wears a mold upon extrusion, undesirably considerably shortening the lifetime thereof, and also because silicon carbide is sintered at a high temperature equal to or higher than 2000° C., undesirably increasing the cost of preparation.
The porous silicon nitride based materials which have superior heat resistance, mechanical properties and corrosion resistance as mentioned above are considered to be promising for use in filters, catalyst supports, heat insulating materials, filters for high-temperature and high-pressure gas, and diesel particulate filters, in lieu of silicon carbide based materials.
However, research into silicon nitride based materials is mainly focused on making the microstructure thereof dense so as to enhance thermo-mechanical properties, and thus methods of manufacturing the porous silicon nitride based materials have not yet been sufficiently studied to date.
As one example of the techniques for manufacturing porous silicon nitride ceramics, there is Korean Unexamined Patent Publication No. 1995-702510 which discloses a method of manufacturing porous silicon nitride ceramics composed of silicon nitride (Si3N4) and a rare earth element compound and/or a transition metal compound so as to be used as a filter for removing impurities or a catalyst support. According to this method, a compact of mixture powder is thermally treated at temperature equal to or higher than 1500° C., thus manufacturing the porous ceramic having a porosity of 30% or more.
In addition, Korean Patent No. 10-0311694 discloses a method of manufacturing porous sintered silicon oxynitride ceramics adapted for the refractory tiles of space shuttles. This method includes agglomerating a low-melting-point powder composition composed of 11˜16 wt % of Si3N4, 3˜5 wt % of AlN, 35˜45 wt % of Al2O3 and 35˜45 wt % of Y2O3, adding 10˜25 wt % of the agglomerated low-melting-point powder to β-Sialon silicon oxynitride powder composed of 57˜100 wt % of Si3N4, 0˜9 wt % of Al2O3 and 0˜33 wt % of AlN, compacting this powder mixture, and sintering the compact at 1600˜1700° C. for 1˜8 hours, thus obtaining the porous sintered silicon oxynitride ceramics.
In addition, Japanese Unexamined Patent Publication No. Hei. 9-100179 discloses a method of manufacturing a porous silicon nitride ceramics usable as a filter or a catalyst support. This method includes bringing the porous ceramic composed mainly of silicon nitride into contact with an acid and/or an alkali so that part or all of the components other than silicon nitride are dissolved, thus manufacturing the porous ceramic.
However, because all the above methods use the expensive silicon nitride, the actual use thereof is basically limited, and also, methods used to form pores are not practical. For example, in the case of Korean Patent No. 10-311694, in order to form pores in the sintered ceramic, the low-melting-point powder composition is compacted into agglomerates, after which the compact thus obtained is mixed with the high-melting-point powder composition, so that the pores are ensured depending on the size of the agglomerated compact. Upon mixing, however, it is difficult for the compact to maintain its shape. If the shape of the compact aims to be maintained, sufficient mixing is impossible. Moreover, it is difficult to consistently control the manufacturing process, and the manufacturing cost cannot but increase. Also, as in Japanese Unexamined Patent Publication No. Hei. 9-100179, the pore formation method which includes chemically treating the manufactured porous ceramic requires additional chemical treatment, and furthermore, if the components between the silicon nitride particles are dissolved, whether the structure of the ceramic is maintained by the silicon nitride backbone cannot be ensured.